Tmux

tmux new-session -d -s python "python3 server.py"

Best practices for session names:

• Use descriptive names: devserver, test-runner, watch-build
• Avoid spaces and special characters
• Consider prefixing with project name if managing multiple projects

Sending Commands to Sessions

Send input to a running session:

tmux send-keys -t <session-name> "<command>" C-m

The C-m sends a carriage return (equivalent to pressing Enter).

Examples:

# Send a restart message
tmux send-keys -t devserver "echo 'restarting...'" C-m

# If 'rs' is a shell alias for restart
tmux send-keys -t devserver "rs" C-m

Capturing Output

Capture the current pane content:

tmux capture-pane -t <session-name> -p

Options:

• -p: Print to stdout (instead of internal buffer)
• -S -: Capture entire scrollback history
• -e: Include escape sequences (for colors)
• -N <lines>: Capture specific number of lines from bottom

Examples:

# Capture visible content
tmux capture-pane -t devserver -p

# Capture full history
tmux capture-pane -t devserver -S - -p

# Capture last 50 lines
tmux capture-pane -t devserver -N 50 -p

Listing Sessions

tmux list-sessions
# or
tmux ls

Output format: <session-name>: <windows> (created <timestamp>) [attached]

Checking Session Status

Check if a specific session exists:

tmux has-session -t <session-name>

Returns exit code 0 if exists, 1 if not. Useful in scripts.

Killing Sessions

Terminate a session:

tmux kill-session -t <session-name>

Kill all sessions:

tmux kill-server

Caution: kill-server terminates all tmux sessions system-wide.

Advanced Usage

Multiple Windows in a Session

Create a new window in an existing session:

tmux new-window -t <session-name> -n <window-name> "<command>"

List windows in a session:

tmux list-windows -t <session-name>

Targeting Specific Panes

By default, commands target pane 0 of the current window. To target specific panes:

tmux send-keys -t <session-name>:<window>.<pane> "<command>" C-m
tmux capture-pane -t <session-name>:<window>.<pane> -p

Example:

# Captures pane 2 of window 1
tmux capture-pane -t devserver:1.2 -p

Environment Variables

Set environment variables for a session:

tmux new-session -d -s mysession "ENV_VAR=value npm run dev"

Common Patterns

Pattern 1: Start, Monitor, Cleanup

# Start a dev server
tmux new-session -d -s devserver "npm run dev"

# Wait a bit, then check output
sleep 5
tmux capture-pane -t devserver -p

# Later, when done
tmux kill-session -t devserver

Pattern 2: Conditional Start

# Only start if session doesn't exist
if ! tmux has-session -t devserver; then
    tmux new-session -d -s devserver "npm run dev"
fi

Pattern 3: Run Command, Get Output

# Send command and immediately capture output
tmux send-keys -t devserver "npm test" C-m
sleep 10
tmux capture-pane -t devserver -N 100 -p | grep -A 20 "Tests:"

Terminating Sessions and Processes

Important: Killing Sessions Doesn't Stop Processes

Important Note: When you kill a tmux session using tmux kill-session, the child processes started within that session are not automatically terminated. They continue running in the background, still holding ports and consuming resources.

To properly stop a long-running process:

1. Send Ctrl+C to the session (recommended):

   tmux send-keys -t <session-name> C-c
   

2. Kill the process directly by PID:

   # Find the PID first
   ps aux | grep <process-name>
   # Then kill it
   kill <PID>
   

3. Kill all processes by name:

   pkill <process-name>
   

Verifying Process Termination

Always verify that processes have been terminated:

# Check if the process is still running
ps aux | grep <process-name>

# Check if the port is still in use
lsof -i :<port>

# Try to connect to verify
curl http://localhost:<port>/health

Example: Properly Stopping a Web Server

# Start a server in tmux
tmux new-session -d -s webserver "npm run dev"

# Later, when you want to stop it:
# DON'T just: tmux kill-session -t webserver
# INSTEAD do:

# Step 1: Send Ctrl+C
tmux send-keys -t webserver C-c

# Step 2: Verify the process is stopped
ps aux | grep node
lsof -i :8080

# Step 3: Kill the session
tmux kill-session -t webserver

This behavior occurs because when you kill a tmux session, it terminates the shell session but doesn't automatically send termination signals to child processes. The processes are children of the shell, not of tmux itself, so they continue running independently.

Troubleshooting

Session Already Exists

If creating a session that exists, tmux will error. Check first:

tmux has-session -t <name> && echo "exists" || echo "free"

No Output in capture-pane

Ensure the session is actually producing output. Some processes buffer output unless they detect a TTY. Forcing PTY allocation is not directly available via tmux CLI.

Session Not Responding

Check if the process hung inside the session. Use tmux capture-pane to see the last output. If necessary, use kill-session to terminate.

Configuration

For programmatic use, consider creating ~/.tmux.conf:

# Disable status bar for cleaner output capture
set -g status off

# Increase scrollback buffer for more history
set -g history-limit 10000

This is optional but helps when capturing large amounts of output.